An image captured by a general digital camera has a limited dynamic range that expresses tonal gradation, and thus it is difficult to express a high dynamic range. To overcome such a limitation, an image enhancement method using characteristics of an infrared image that cannot be perceived by humans has been proposed.
Methods disclosed in “Image enhancement through weighting function estimation with infrared image (J. S. Kim et al., 2010),” “Merging thermal and visual images by a contrast pyramid (Toet a et al., 2007),” etc., are based on merging of an image captured in an infrared region and an image captured in a visible region. The merging uses a characteristic in which a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) sensor of a general digital camera can collect information on a larger region than visible rays.
Specifically, a wavelength of a near-infrared region ranges from 700 nm to 1200 nm, which is longer than a wavelength of the visible region ranging from 350 nm to 750 nm. Therefore, infrared rays are less scattered and thus have an advantage in terms of transmission. Also, infrared images include useful information that is not seen in the visible region. For these reasons, when information included in an infrared image is used for image enhancement, it is possible to process an image properly.
However, such a method of merging an infrared image and a visible image has some problems. First, a process of merging two images involves pixel-to-pixel alignment, which may cause unavoidable blurring. Also, an exposure time increases due to light blocking by an infrared filter of a camera, and noise may occur in an image. Further, it is not easy to set a focus for capturing an infrared image.
As described above, information included in an infrared image is useful to enhance an image, but there are several problems in a method of additionally capturing the infrared image and merging the images together. Therefore, there is a need for a new method to replace the existing method.